The present invention relates to a laser sensor for detecting and counting flying particles or drops, using a small diameter laser beam, such as a sensor for detecting ink drops in an ink jet type printer. More particularly, the present invention relates to a laser sensor using a laser beam, which has a substantially constant beam diameter in a long detecting section, so that a stable detection can be realized.
Such a sensor for detecting, for example, ink drops, comprises a laser source, a condenser lens and a light detecting portion. The laser source and the light detecting portion are arranged so that they face to each other. The laser beam is narrowed by the condenser lens to be a small diameter beam. When a particle enters into an area of the narrow beam, the laser beam is intercepted, therefore the particle can be detected according to the change of the light quantity received by the light detecting portion. In general, the particles, which will pass the laser beam have a diameter of about some ten micro meters, and it is preferable that the laser beam is narrowed to be less than 10 times of the diameter of the particles. Further, a long diameter narrowed section of the laser beam is desirable for securing a long detection section in an ink jet type printer so that particles can be detected, irrespective to the change of the position of an ink jet cartridge in an ink jet type printer due to the attaching allowance. In the prior art, three types of laser beam narrowing methods are used: (1) Parallel beam method; (2) Parallel beam and pin hole method; and (3) Converging beam method.
(1) Parallel beam method: The laser beam from a generator, such as gas laser, is used as is generated. The diameter of a laser beam emitting from a gas laser generator can be made small, by making the length of the gas laser resonator shorter. When the diameter of a laser beam is small, however, the diversity of the laser beam increases, as shown in FIG. 13, which shows the relation between the beam diameter at a distance of 100 mm from the laser generator and the beam diameter at the outlet of the laser generator. Therefore, there is a limitation to obtain a small diameter laser beam at a far distance from the laser source.
(2) Parallel beam and pin hole method: The optical system composed of a combination of a parallel beam and a pin hole is as follows; the laser source is disposed at a focus point of a condenser lens, and the parallel beam from the condenser lens is used, and a pin hole having a desired beam diameter is disposed in the back side of the lens. By decreasing the diameter of the pin hole, the diameter of the laser beam can be decreased. However, when the pin hole diameter is less than a certain value depending on the laser wave length, the diameter of the laser beam increases rapidly, due to the diffraction effect as FIG. 14 shows the relation between the beam diameter at a distance of 100 mm from the lens and the pin hole diameter. Therefore, it is difficult to obtain a small diameter laser beam having a constant beam diameter over a long section, which is longer than a certain distance.
(3) Converging beam method: In this method, the diameter of the laser beam is narrowed by means of a lens. According to this method, it is possible to obtain a minimum diameter spot, which is given by the expression d=1.22 xcex/NA. Therefore, the diameter of the laser beam can be decreased to an order of the laser wave length (xcex) by increasing the numerical aperture NA of the lens. According to this method, however, the beam diameter rapidly increases when it exceeds the depth of focus=xcex/(NA)2. Namely, the beam diameter greatly varies depending on the distance from the lens, as shown in FIG. 15, which shows the relation between the beam diameter and the distance from the lens. Therefore, when such a sensor is used as a particle sensor, it is very difficult to determine a reference light detection level for deciding whether a particle is detected or not.
According to any of the aforementioned prior arts, it is impossible to obtain a small diameter laser beam, having a constant diameter over a long section, which is longer than 100 mm, for example. Therefore, it is recognized as a problem that it is difficult to judge whether a constant quantity of ink drops is supplied, using a constant diameter of laser beam, when a small change of the ink dropping position occurs by the exchange of an ink cartridge exchange.
Furthermore, according to any of the aforementioned prior arts, the beam converging point, namely the focusing point, depends strongly on the focal length of the condenser lens. Thus a lens made from a material, the refraction index of which depends strongly on the temperature, such as plastics, can not be used. And the lens must be made from a material, the refraction index of which hardly depends on the temperature, such as glass. In such an optical system, an aspherical lens is used, however, a glass aspherical lens is hard to fabricate commercially. Thus the cost of such a sensor is very expensive.
An object of the present invention is to solve those problems in such sensors in the prior art, providing a laser sensor enabling to obtain a small diameter laser beam, having a constant diameter over a long section, which is longer than 100 mm, for example, irrespective to a small change of the detecting position.
Another object of the present invention is to provide a laser sensor, which allows to use a plastics aspherical lens which also enables that the narrowed beam converging position hardly changes even when the focal length of the condenser lens changes a little.
The inventor of the present invention made repeated reviews in order to realize a laser sensor which can maintain a small and constant beam diameter over a long section longer than about 100 mm as mentioned above. As a result, it is found that a smaller diameter beam having a constant beam diameter over a long section over a certain distance can be obtained by a condenser lens converging a laser beam and disposing a pin hole, which has a rather large diameter in the converging beam passes at the condenser lens side of the converging beam. A conventional pin hole is disposed at the converging point of converging beam, or, as shown in the above-mentioned (2), is disposed in a parallel beam such that it passes through all or a part of the parallel beam. On the other hand, in the present invention, the light is converged by a condenser lens, and a rather large diameter pin hole is disposed in the converging light beam, so that a small diameter beam having a constant diameter over a section of certain length can be obtained. The inventor further studied and found that there is an appropriate range with respect to the position of the pin hole plate, the distance between the converging point of the condenser lens and the condenser lens and that, by optimizing them, a further smaller diameter beam having a constant beam diameter over a longer section can be obtained.
The laser sensor, according to the present invention comprises: a laser source; a condenser lens for converging the light from the laser source; a pin hole plate disposed adjacent to the condenser lens in the opposite side to the laser source, the pin hole plate having a pin hole positioned in the converging beam converged by the condenser lens; and a light detecting portion disposed so as to face to the converging beam passed the pin hole, wherein the diameter of the pin hole is formed to be 0.4 to 0.7 mm.
The expression adjacent to the condenser lens does not mean adjacent to the converging point (image point) of the converging beam, but means adjacent to the condenser lens or contact with the condenser lens, and it is preferable to have it nearer to the condenser lens. The term pin hole means a small hole disposed in a portion of a light blocking plate so as to pass a light therethrough.
Employing such configuration, it becomes possible to maintain the diameter of a small diameter beam to be constant over a long section and to realize a laser sensor which allows to detect particles exactly, even when the longitudinal position of the particles along the light beam can not be completely determined. The reason why the small diameter beam can be maintained over a long section is quite likely that the diameter of the light beam is determined according to the diffraction effect of a pin hole provided in the converging beam, which is disposed at a point adjacent to the condenser lens at the back side of the condenser lens.
It is preferable that the pin hole plate is disposed at the back side of the condenser lens, and the distance between the pin hole plate and the back face of the condenser lens is less than ⅕ of the distance sxe2x80x3 between the image point of the laser source by the condenser lens and the second principal point of the condenser lens.
It is preferable that the position of the laser source and the focal length of the condenser lens are designed so that the distance sxe2x80x3 between the image point of the laser source by the condenser lens and the second principal point of the condenser lens is larger than a certain required distance xcex94z0, in which the predetermined beam diameter is maintained. Here, certain required distance, in which the predetermined beam diameter is maintained, means a length of a section in which the flying particles shall be detected, and the diameter of the light beam shall be constant. The necessary length of the section depends on each of the applications.
A condenser lens comprised of a plastics aspherical lens is preferable, because such a lens is easy to fabricate and the fabrication cost can be reduced. It allows to obtain a small diameter light beam having a constant diameter over a long section, by disposing a suitable pin hole plate to such a plastics aspherical lens.